Pigment Dispersion with Polymeric Dispersants Having Pending Chromophore Groups

ABSTRACT

A pigment dispersion includes a color pigment represented by formula (I): 
     
       
         
         
             
             
         
       
         
         
           
             wherein 
             X1 to X4 are independently selected from the group consisting of hydrogen and a halogen atom; 
             R1 to R10 are independently selected from the group consisting of hydrogen, a halogen atom, a methyl group, an ethyl group, a methoxy group, and an ethoxy group; 
             and a polymeric dispersant, having via a linking group covalently linked to its polymeric backbone, at least one pending chromophore group which has a molecular weight smaller than 90% of the molecular weight of the color pigment; wherein the at least one pending chromophore group is a chromophore group occurring as a side group on the polymeric backbone and not a group in the polymeric backbone itself or occurring solely as an end group of the polymeric backbone; 
             and the at least one pending chromophore group is represented by formula (II): 
           
         
       
    
     
       
         
         
             
             
         
       
         
         
           
             wherein 
             one of L1, L2, or L3 is the linking group and is selected from the group consisting of an aliphatic group, a substituted aliphatic group, an unsaturated aliphatic group, and a substituted unsaturated aliphatic group; 
             L1, L2, and/or L3, if not representing the linking group, are independently selected from the group consisting of hydrogen, an alkyl group, an alkenyl group, an alkoxy group, a carboxylic acid group, an ester group, an acyl group, a nitro group, and a halogen; 
             AR1 and AR2 represent an aromatic group; and 
             n represents the integer 0 or 1. The pigment dispersion can be advantageously used in inkjet inks. Also disclosed are methods for preparing the inkjet ink.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 of PCT/EP2006/063484, filed Jun. 23, 2006.This application claims the benefit of U.S. Provisional Application No.60/712,503, filed Aug. 30, 2005, which is incorporated herein byreference in its entirety. In addition, this application claims thebenefit of European Application No. 05106454.1, filed Jul. 14, 2005,which is also incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to stable pigment dispersions andpigmented inkjet inks including color pigments that are stabilized bypolymeric dispersants having pending chromophore groups which exhibit astructural similarity with the color pigment.

2. Description of the Related Art

Pigment dispersions are made using a dispersant. A dispersant is asubstance for promoting the formation and stabilization of a dispersionof pigment particles in a dispersion medium. Dispersants are generallysurface-active materials having an anionic, cationic, or non-ionicstructure. The presence of a dispersant substantially reduces thedispersing energy required. Dispersed pigment particles may have atendency to re-agglomerate after the dispersing operation, due to mutualattraction forces. The use of dispersants also counteracts thisre-agglomeration tendency of the pigment particles.

The dispersant has to meet particularly high requirements when used forinkjet inks. Inadequate dispersing manifests itself as increasedviscosity in liquid systems, loss of brilliance, and/or hue shifts.Moreover, particularly good dispersion of the pigment particles isrequired to ensure unimpeded passage of the pigment particles throughthe nozzles of the print head, which are usually only a few micrometersin diameter. In addition, pigment particle agglomeration and theassociated blockage of the printer nozzles has to be avoided in thestandby periods of the printer.

Polymeric dispersants contain in one portion of the molecule so-calledanchor groups, which adsorb onto the pigments to be dispersed. In aspatially separate portion of the molecule, polymeric dispersants havepolymer chains sticking out whereby pigment particles are madecompatible with the dispersion medium, i.e., are stabilized.

The properties of polymeric dispersants depend on both the nature of themonomers and their distribution in the polymer. Polymeric dispersantsobtained by randomly polymerizing monomers (e.g., monomers A and Bpolymerized into ABBAABAB) or by polymerizing alternating monomers(e.g., monomers A and B polymerized into ABABABAB) generally result in apoor dispersion stability. Improvements in dispersion stability havebeen obtained using graft copolymer and block copolymer dispersants.

Graft copolymer dispersants consist of a polymeric backbone with sidechains attached to the backbone. CA 2157361 (DU PONT) discloses pigmentdispersions made by using a graft copolymer dispersant with ahydrophobic polymeric backbone and hydrophilic side chains.

Block copolymer dispersants containing hydrophobic and hydrophilicblocks have been disclosed in numerous inkjet ink patents. U.S. Pat. No.5,859,113 (DU PONT) discloses an AB block copolymer dispersant with apolymeric A segment of polymerized glycidyl (meth)acrylate monomersreacted with an aromatic or aliphatic carboxylic acid, and a polymeric Bsegment of polymerized alkyl (meth)acrylate monomers having 1-12 carbonatoms in the alkyl group, hydroxy alkyl(meth)acrylate monomers havingabout 1-4 carbon atoms in the alkyl group.

In the design of polymeric dispersants for aqueous inkjet inks, theabove mentioned anchor groups, which adsorb onto the pigments to bedispersed, are generally hydrophobic groups exhibiting an affinity forthe pigment surface.

EP 0763580 A (TOYO INK) discloses an aqueous type pigment dispersingagent having a portion which has a high affinity with a pigment andwhich has at least one type selected from the group consisting of anorganic dye, anthraquinone, and acridone only at a terminal end or atboth terminal ends of at least one aqueous polymer selected from thegroup consisting of an aqueous linear urethanic polymer and an aqueouslinear acrylic polymer. EP 0763378 A (TOYO INK) discloses similarpigment dispersing agents for non-aqueous pigment dispersions.

U.S. Pat. No. 5,420,187 (TOYO INK) discloses a pigment-dispersing agentobtained by polymerizing an addition-polymerizable monomer having anacidic functional group and other addition-polymerizable monomer in thepresence of a polymerization initiator, the polymerization initiatorbeing a diazotization product prepared by diazotizing at least onecompound selected from the group consisting of an anthraquinonederivative having an aromatic amino group, an acridone derivative havingan aromatic amino group, and an organic dyestuff having an aromaticamino group. In this pigment-dispersing agent, the colorant is locatedin the polymeric backbone itself.

U.S. 2003/0044707 (TOYO INK) discloses a dispersing agent for a pigment,including a specific compound having a structure wherein aphthalocyanine type molecular skeleton which is adsorptive on thepigment and an oligomer unit or polymer unit which preventsre-agglomeration of the pigment to bring out the effect of dispersionare covalently bonded, and having affinity for a medium or a solvent.

U.S. 2004/0194665 (BASF) relates to pigment dispersions including for apigment dispersant a substituted perylene derivative in which thesubstituent has a sterically stabilizing and/or electrostaticallystabilizing effect. The perylene derivatives are used to dispersepigments with a quite different chemical structure such as quinacridonepigments.

WO 2005/056692 (SUN CHEMICAL) discloses highly concentrated colorantdispersions including: (a) at least about 45 wt. % of a pigment; and (b)a polymeric colored dispersant having the structure A-(B—X), wherein: Ais an organic chromophore; B is a covalently bonded linking moiety; X isa branched or linear C₅₀-C₂₀₀ polymeric covalently linked hydrocarbon;and n is an integer from 1 to 4. The chromophore group A is present inthe polymeric dispersant either as an end group or in the polymericbackbone. The polymeric dispersants are used for dispersing pigmentswith quite different chemical structure compared to the chromophoregroup A.

U.S. Pat. No. 4,664,709 (HOECHST) discloses pigment compositionsincluding azoacylacetamide dispersing agents for azo pigments bearingsome resemblance to the chromophore group of the dispersing agent. Thechromophore group is present in the polymeric dispersant as an endgroup.

GB 1343606 (ICI) discloses pigment dispersions containing a dyestuff ofthe formula D-(Z-O.OCR)_(n) in which D is the radical of a dyestuffwhich is attached to Z through a carbon atom of an aromatic ring presentin D, Z is a divalent bridging group, n is an integer of 1-8, and R isthe residue of a carboxy ended polyester RCOOH derived from ahydroxycarboxylic acid of the formula HO—X—COOH in which X is a divalentsaturated or unsaturated aliphatic radical containing at least 8 carbonatoms and in which there are at least 4 carbon atoms between the hydroxyand carboxy acid groups, or from a mixture of such a hydroxycarboxylicacid and a carboxylic acid free from hydroxy groups. In the examples,the pigments are dispersed with dyestuffs having a radical D ofapproximately the same or higher molecular weight compared to thepigment. The radical of a dyestuff D is present in the polymericdispersant either as an end group or in the polymeric backbone.

Current practice is that the exact or almost the exact chemicalstructure of the color pigment is incorporated as the anchor group inthe polymeric dispersing agent to assure maximum affinity with the colorpigment. As a consequence, each pigment has its own tailor-madepolymeric dispersant. In practice, this requires the holding of aninventory of different polymeric dispersants for producing a completerange of color inkjet ink sets. The cyan ink with copper phthalocyanineas the pigment is a rare exception in that all desired properties arecombined in the same pigment. But yellow pigments have to be selectedbased on the properties that are the most important in their applicationof the inkjet ink. For example, some yellow pigments are selected fortheir light stability, while others are selected to obtain images havinghigh color strength. The holding of such an inventory of different typesof polymeric dispersants incurs financial penalties due to additionalstorage and logistical requirements as well as increasing thepossibility of using the “wrong” polymeric dispersant for the productionof a particular inkjet ink. Another disadvantage is that the lowsolubility of the pigment generally complicates the synthesis of suchpolymeric dispersants.

For consistent image quality, the inkjet ink requires a dispersionstability capable of dealing with high temperatures (above 60° C.)during transport of the ink to a customer and changes in the dispersionmedium of the inkjet ink during use, for example, evaporation of solventand increasing concentrations of humectants, penetrants, and otheradditives.

Therefore, it is highly desirable to be able to manufacture a range ofstable pigmented inkjet inks using a single polymeric dispersantobtained by simple synthesis.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide inkjet inks using a polymericdispersant obtainable by uncomplicated synthesis and suitable fordifferent color pigments.

Further preferred embodiments of the present invention provide inkjetinks with high dispersion stability.

Further preferred embodiments of the present invention provide inkjetinks producing images of high image quality with a high optical density.

Further preferred embodiments of the present invention will becomeapparent from the description hereinafter.

It has been surprisingly discovered that inkjet inks with high opticaldensity and high stability can be obtained using a colored polymericdispersant wherein a pending chromophore group exhibits a structuralsimilarity with the color pigment, but is smaller in size than the colorpigment.

A further preferred embodiment of the present invention has beenachieved with a pigment dispersion including a color pigment representedby formula (I):

whereinX1 to X4 are independently selected from the group consisting ofhydrogen and a halogen atom;R1 to R10 are independently selected from the group consisting ofhydrogen, a halogen atom, a methyl group, an ethyl group, a methoxygroup, and an ethoxy group;and a polymeric dispersant having, via a linking group covalently linkedto its polymeric backbone, at least one pending chromophore group whichhas a molecular weight smaller than 90% of the molecular weight of thecolor pigment;the at least one pending chromophore group is a chromophore groupoccurring as a side group on the polymeric backbone and not a group inthe polymeric backbone itself or occurring solely as an end group of thepolymeric backbone;and the at least one pending chromophore group is represented by formula(II):

whereinone of L1, L2, or L3 is the linking group and is selected from the groupconsisting of an aliphatic group, a substituted aliphatic group, anunsaturated aliphatic group, and a substituted unsaturated aliphaticgroup;L1, L2, and/or L3, if not representing the linking group, areindependently selected from the group consisting of hydrogen, an alkylgroup, an alkenyl group, an alkoxy group, a carboxylic acid group, anester group, an acyl group, a nitro group, and a halogen;AR1 and AR2 represent an aromatic group; andn represents the integer 0 or 1.

Other features, elements, processes, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of preferred embodiments of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Definitions

The term “colorant”, as used in the preferred embodiments of the presentinvention, means dyes and pigments.

The term “dye”, as used in the preferred embodiments of the presentinvention, means a colorant having a solubility of 10 mg/L or more inthe medium in which it is applied and under the ambient conditionspertaining thereto.

The term “pigment” is defined in DIN 55943, herein incorporated byreference, as a coloring agent that is practically insoluble in theapplication medium under the pertaining ambient conditions, hence havinga solubility of less than 10 mg/L therein.

The term “chromophore group”, as used in the preferred embodiments ofthe present invention, means a group with an absorption maximum between300 nm and 2000 nm.

The term “pending chromophore group”, as used in the preferredembodiments of the present invention, means a chromophore groupoccurring as a side group on the polymeric backbone and not a group inthe polymeric backbone itself or occurring solely as an end group of thepolymeric backbone.

The term “C.I.” is used in the preferred embodiments of the presentapplication as an abbreviation for Color Index.

The term “actinic radiation” as used in the preferred embodiments of thepresent invention, means electromagnetic radiation capable of initiatingphotochemical reactions.

The term “DP” is used in the preferred embodiments of the presentapplication as an abbreviation for degree of polymerization, i.e., thenumber of structural units (monomers) in the average polymer molecule.

The term “PD” is used in the preferred embodiments of the presentapplication as an abbreviation for polydispersity of a polymer.

The term “dispersion”, as used in the preferred embodiments of thepresent invention, means an intimate mixture of at least two substances,one of which, called the dispersed phase or colloid, is uniformlydistributed in a finely divided state through the second substance,called the dispersion medium.

The term “polymeric dispersant”, as used in the preferred embodiments ofthe present invention, means a substance for promoting the formation andstabilization of a dispersion of one substance in a dispersion medium.

The term “copolymer”, as used in the preferred embodiments of thepresent invention means a macromolecule in which two or more differentspecies of monomer are incorporated into a polymer chain.

The term “block copolymer”, as used in the preferred embodiments of thepresent invention, means a copolymer in which the monomers occur inrelatively long alternate sequences in a chain.

The term “spectral separation factor” as used in the preferredembodiments of the present invention means the value obtained bycalculating the ratio of the maximum absorbance A_(max) (measured atwavelength λ_(max)) over the reference absorbance A_(ref) determined ata higher wavelength λ_(ref).

The abbreviation “SSF” is used in the preferred embodiments of thepresent invention for spectral separation factor.

The term “alkyl” means all variants possible for each number of carbonatoms in the alkyl group, i.e., for three carbon atoms: n-propyl andisopropyl; for four carbon atoms: n-butyl, isobutyl, and tertiary-butyl;for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl,2,2-dimethylpropyl, and 2-methyl-butyl etc.

The term “acyl group” means —(C═O)-aryl and —(C═O)-alkyl groups.

The term “aliphatic group” means saturated straight chain, branchedchain, and alicyclic hydrocarbon groups.

The term “unsaturated aliphatic group” means straight chain, branchedchain, and alicyclic hydrocarbon groups which contain at least onedouble or triple bond.

The term “aromatic group” as used in the preferred embodiments of thepresent invention means an assemblage of cyclic conjugated carbon atoms,which are characterized by large resonance energies, e.g., benzene,naphthalene and anthracene.

The term “alicyclic hydrocarbon group” means an assemblage of cycliccarbon atoms, which do not form an aromatic group, e.g., cyclohexane.

The term “heteroaromatic group” means an aromatic group wherein at leastone of the cyclic conjugated carbon atoms is replaced by a non-carbonatom such as a nitrogen atom, a sulphur atom, a phosphorous atom, aselenium atom, and a tellurium atom.

The term “heterocyclic group” means an alicyclic hydrocarbon groupwherein at least one of the cyclic carbon atoms is replaced by an oxygenatom, a nitrogen atom, a phosphorous atom, a silicon atom, a sulphuratom, a selenium atom, or a tellurium atom.

Pigmented Inkjet Ink

The pigmented inkjet ink according to a preferred embodiment of thepresent invention contains at least three components: (i) a colorpigment, (ii) a polymeric dispersant, and (iii) a dispersion medium.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may further contain at least one surfactant.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may further contain at least one biocide.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may further contain at least one humectant and/orpenetrant.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may further contain at least one pH adjuster.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may contain at least one humectant to prevent theclogging of the nozzle due to its ability to slow down the evaporationrate of ink.

The viscosity of the pigmented inkjet ink according to a preferredembodiment of the present invention is preferably lower than 100 mPa·s,more preferably lower than 30 mPa·s, and most preferably lower than 15mPa·s at a shear rate of 100 s⁻¹ and a temperature between 20° C. and110° C.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention is preferably an aqueous, solvent based, or an oilbased pigmented inkjet ink.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may be curable and may contain monomers, oligomers,and/or prepolymers possessing different degrees of functionality. Amixture including combinations of mono-, di-, tri- and/or higherfunctionality monomers, oligomers, or prepolymers may be used. Theinitiator typically initiates the polymerization reaction. A catalystcalled an initiator for initiating the polymerization reaction may beincluded in the curable pigmented inkjet ink. The initiator can be athermal initiator, but is preferably a photo-initiator. Thephoto-initiator requires less energy to activate than the monomers,oligomers, and/or prepolymers to form the polymer. The photo-initiatorsuitable for use in the curable fluid may be a Norrish type I initiator,a Norrish type II initiator, or a photo-acid generator

Color Pigments

The color pigment used in the pigmented inkjet ink according to apreferred embodiment of the present invention is represented by formula(I):

whereinX1 to X4 are independently selected from the group consisting ofhydrogen and a halogen atom; andR1 to R10 are independently selected from the group consisting ofhydrogen, a halogen atom, a methyl group, an ethyl group, a methoxygroup, and an ethoxy group.

In a preferred embodiment, X1 to X4 are independently selected from thegroup consisting of hydrogen and chlorine.

In another preferred embodiment, X1 and X3 are chlorine and X2 and X4are hydrogen.

In another preferred embodiment, X1 to X4 are chlorine.

In another preferred embodiment, R1, R4, R7, and R10 are hydrogen and R3and/or R8 are a methyl group or a methoxy group.

In another preferred embodiment, R1, R4, R7, and R10 are hydrogen and R5and/or R6 are a methyl group or a methoxy group.

The color pigment may be chosen from those disclosed by HERBST et al.,Industrial Organic Pigments, Production, Properties, Applications; 3rdEdition, Wiley-VCH, 2004, ISBN 3527305769.

Particular preferred pigments are C.I. Pigment Yellow 12, 13, 14, 17,55, 63, 81, 83, 87, 113, 121, 124, 152, 170, 171, 172, 174, and 188.

The pigment particles in the pigmented inkjet ink should be sufficientlysmall to permit free flow of the ink through the inkjet printing device,especially at the ejecting nozzles. It is also desirable to use smallparticles for maximum color strength and to slow down sedimentation.

The average particle size of the pigment in the pigmented inkjet inkshould be between 0.005 μm and 15 μm. Preferably, the average pigmentparticle size is between 0.005 μm and 5 μm, more preferably between0.005 μm and 1 μm, particularly preferably between 0.005 μm and 0.3 μm,and most preferably between 0.040 μm and 0.150 μm. Larger pigmentparticle sizes may be used as long as the advantages of the presentinvention are achieved.

The pigment is used in the pigmented inkjet ink in an amount of 0.1 wt %to 20 wt %, preferably 1 wt % to 10 wt % based on the total weight ofthe pigmented inkjet ink.

Polymeric Dispersants

The polymeric dispersant used in the pigmented inkjet ink according to apreferred embodiment of the present invention contains one or morepending chromophore groups linked by a linking group to a polymericbackbone.

The polymeric dispersant used in the pigmented inkjet ink according to apreferred embodiment of the present invention preferably has a polymericbackbone with a polymerization degree DP between 5 and 1,000, morepreferably between 10 and 500, and most preferably between 10 and 100.

The polymeric dispersant used in the pigmented inkjet ink according to apreferred embodiment of the present invention preferably has a numberaverage molecular weight Mn between 500 and 30,000, more preferablybetween 1,500 and 10,000.

The polymeric dispersant preferably has a polymeric dispersity PDsmaller than 2, more preferably smaller than 1.75, and most preferablysmaller than 1.5.

The polymeric dispersant is preferably used in the pigmented inkjet inkin an amount of 5 wt % to 600 wt %, preferably 10 wt % to 100 wt % basedon the weight of the pigment.

Polymeric Backbones

The polymeric backbone of the polymeric dispersant used in the pigmentedinkjet ink according to a preferred embodiment of the present inventionis required for the compatibility between polymeric dispersant anddispersion medium.

It is not required that the polymeric backbone has an affinity for thepigment. For example, the polymeric backbone of a dispersant for aqueousinkjet inks can be a homopolymer of acrylic acid monomers. A homopolymeris generally incapable of dispersing pigments, but the presence of apending chromophore group exhibiting a similarity with the pigmentensures an adequate affinity between polymeric dispersant and pigmentsurface.

The polymeric backbone can also be a statistical copolymer, a blockcopolymer, a graft copolymer, a comb polymer, or an alternatingcopolymer. Also suitable as polymeric backbone is a gradient copolymeras disclosed by MATYJASZEWSKI et al., Atom Transfer RadicalPolymerization, Chem. Reviews 2001, Vol. 101, pp. 2921-2990. Sometimesit can be useful to include a number of monomers with a high affinityfor the pigment surface to enhance certain properties of the inks, e.g.,dispersion stability. For example, the polymeric backbone of adispersant for aqueous inkjet inks may contain hydrophobic monomers toincrease the affinity of the polymeric dispersant for the pigmentsurface. However, in enhancing this affinity for the pigment surface,care should be taken that enough of the polymeric backbone sticks out tomake the pigment particles compatible with the dispersion medium.

In graft copolymers the use of grafted chains ofmethoxypolyethyleneglycol (MPEG) has been found to be very advantageousin aqueous inkjet inks. For solvent-based inkjet inks the use of graftedchains of polyester were found to be very advantageous. A preferred MPEGmacromonomer is BISOMER™ MPEG 350MA (methoxypolyethyleneglycolmethacrylate) from LAPORTE INDUSTRIES LTD.

Preferred grafted chains of polyester in non-aqueous inkjet inks arederived from δ-valerolactone, ε-caprolactone, and/or C₁ to C₄ alkylsubstituted ε-caprolactone. The grafted chains can be introduced intothe polymeric dispersant through CDI coupling of a polyester-OH chainwith a carboxylic acid group of, for example, an acrylic acid monomer inthe polymeric backbone of the dispersant. However, it was observed thatgrafting by free radical polymerization, wherein the polyester chainalready coupled to the carboxylic acid group of an acrylic acid monomerwas used as a macro-monomer, not only resulted in better dispersionquality and stability of the inkjet inks but also were obtained by amore reproducible polymeric dispersant synthesis requiring lesspurification.

For radiation curable inks with the dispersion medium including orconsisting of monomers and/or oligomers, many (co)polymers having goodsolubility in the dispersion medium were found to be suitable for thepolymer backbone of the polymeric dispersant.

The copolymeric backbone consists preferably of no more than 2 or 3monomer species.

The monomers and/or oligomers used to prepare the polymeric dispersantused in the pigmented inkjet ink according to a preferred embodiment ofthe present invention can be any monomer and/or oligomer found in thePolymer Handbook, Vol. 1+2, 4th Edition, edited by J. BRANDRUP et al.,Wiley-Interscience, 1999.

Suitable examples of monomers include: acrylic acid, methacrylic acid,maleic acid, acryloyloxybenzoic acid and methacryloyloxybenzoic acid (ortheir salts), and maleic anhydride; alkyl(meth)acrylates (linear,branched, and cycloalkyl) such as methyl(meth)acrylate,n-butyl(meth)acrylate, tert-butyl(meth)acrylate,cyclohexyl(meth)acrylate, and 2-ethylhexyl(meth)acrylate;aryl(meth)acrylates such as benzyl(meth)acrylate andphenyl(meth)acrylate; hydroxyalkyl(meth)acrylates such ashydroxyethyl(meth)acrylate and hydroxypropyl(meth)acrylate;(meth)acrylates with other types of functionalities (e.g., oxirane,amino, fluoro, polyethylene oxide, and phosphate-substituted) such asglycidyl (meth)acrylate, dimethylaminoethyl(meth)acrylate,trifluoroethyl acrylate, methoxypolyethyleneglycol (meth)acrylate, andtripropyleneglycol(meth)acrylate phosphate; allyl derivatives such asallyl glycidyl ether; styrenics such as styrene, 4-methylstyrene,4-hydroxystyrene, and 4-acetoxystyrene; (meth)acrylonitrile;(meth)acrylamides (including N-mono and N,N-disubstituted) such asN-benzyl(meth)acrylamide; maleimides such as N-phenyl maleimide,N-benzyl maleimide, and N-ethyl maleimide; vinyl derivatives such asvinylcaprolactam, vinylpyrrolidone, vinylimidazole, vinylnaphthalene,and vinyl halides; vinylethers such as vinylmethyl ether; andvinylesters of carboxylic acids such as vinylacetate and vinylbutyrate.

Linking Groups

The pending chromophore group is linked by a linking group to thepolymeric backbone. The linking group contains at least one carbon atom,one nitrogen atom, one oxygen atom, one phosphorous atom, one siliconatom, one sulphur atom, one selenium atom, or one tellurium atom.

The linking groups L1 and L3 in the pending chromophore group accordingto Formula (II) consist of all atoms between the polymeric backbone andthe first atom of the aromatic group by which the pending chromophoregroup is linked to the polymeric backbone. The linking group L2 in thepending chromophore group according to Formula (II) consists of allatoms between the polymeric backbone and the carbon atom directly linkedto both L2 and the carbonyl group of the acetoacetanilide group in thepending chromophore group according to Formula (II).

The linking group has preferably a molecular weight smaller than themolecular weight of the pending chromophore group, more preferably thelinking group has a molecular weight smaller than 80% of the molecularweight of the pending chromophore group, and most preferably the linkinggroup has a molecular weight smaller than 50% of the molecular weight ofthe pending chromophore group.

In a preferred embodiment, the linking group is the result ofmodification of a (co)polymer with a chromophore having a reactivegroup. Suitable reactive groups on the chromophore include thiol groups,primary or secondary amino groups, carboxylic acid groups or saltsthereof, hydroxyl groups, isocyanate groups, and epoxy groups. Typicalcovalent bonds formed by reaction of the chromophore with the polymericbackbone include an amide, an ester, a urethane, an ether, and athioether.

In another preferred embodiment, the polymeric dispersant is prepared bycopolymerizing monomers of the polymeric backbone and monomerscontaining a chromophore group. In this case the linking group isalready present in the monomer. This polymerization method offers theadvantage of well-controlled design of polymeric dispersants for a widevariety of dispersion media. Due to its low solubility, a monomercontaining the complete color pigment as a chromophore group posesproblems both in the synthesis of the polymeric dispersants, as well asthe suitability of the polymeric dispersant for a wide variety ofdispersion media and pigments.

Pending Chromophore Groups

The pending chromophore group of the polymeric dispersant used in thepigmented inkjet ink according to a preferred embodiment of the presentinvention exhibits a high similarity with the color pigment of thepigmented inkjet ink, and has a molecular weight which is smaller than90%, preferably smaller than 85%, more preferably smaller than 75%, andmost preferably smaller than 65% of the molecular weight of the colorpigment.

The pending chromophore group of the polymeric dispersant may berepresented by formula (II):

whereinone of L1, L2, or L3 is the linking group and is selected from the groupconsisting of an aliphatic group, a substituted aliphatic group, anunsaturated aliphatic group, and a substituted unsaturated aliphaticgroup;L1, L2, and/or L3, if not representing the linking group, areindependently selected from the group consisting of hydrogen, an alkylgroup, an alkenyl group, an alkoxy group, a carboxylic acid group, anester group, an acyl group, a nitro group, and a halogen;AR1 and AR2 represent an aromatic group; andn represents the integer 0 or 1.

In a preferred embodiment, the pending chromophore group of thepolymeric dispersant may be represented by formula (III):

whereinone of R1 to R11 is the linking group forming a covalent bond with thepolymeric backbone;R1 to R11, if not representing the linking group, are independentlyselected from the group consisting of hydrogen, an alkyl group, analkenyl group, an alkoxy group, an alcohol group, a carboxylic acidgroup, an ester group, an acyl group, a nitro group, and a halogen; orR7 and R8 may together form a heterocyclic ring. Preferably theheterocyclic ring formed by R7 and R8 is imidazolone or2,3-dihydroxypyrazine, so that a benzimidazolone ring and a2,3-dihydroxyquinoxaline ring respectively are formed in Formula (III).

Suitable examples of the pending chromophore group represented byformula (III) having an unreacted linking group include:

The pending chromophore group is preferably present in the range of 1 to30 percent, more preferably in the range 5 to 20 percent based on themonomeric units of the polymeric backbone. Polymeric dispersants havinga homopolymer or statistical copolymer as polymeric backbone with morethan 45 percent of the monomeric units of the polymeric backbone havingpending chromophore groups exhibit problems of solubility of thepolymeric dispersant in the dispersion medium and deterioration of thedispersing properties due to the fact that the dispersant would go flaton the pigment surface. However, in the case of a well-definedblock-copolymer a good dispersion can be obtained with 50 percent of themonomeric units of the polymeric backbone having pending chromophoregroups. This well-defined block-copolymer preferably has at least oneblock containing no pending chromophore groups.

In some cases, the dispersion stability of the pigment according to apreferred embodiment of the present invention can be further improved byincreasing the number of pending chromophore groups in the polymericdispersant. In a preferred embodiment, two, three, or more pendingchromophore groups are located in close proximity of each other on thepolymeric backbone. Close proximity means preferably less than 50monomeric units, more preferably less than 20 monomeric units, and mostpreferably less than 10 monomeric units between two pending chromophoregroups. It is believed that the dispersion stability improvement by morepending chromophore groups is due to the dynamic character of theattaching and detaching of the pending chromophore group to the pigmentsurface. By increasing the number of pending chromophore groups, theprobability that all pending chromophore groups will be in a “detachedstate” at the same time is expected to decrease.

Synthesis

The polymerization process may be a condensation polymerization, inwhich the chain growth is accompanied by elimination of small moleculessuch as water or methanol or an addition polymerization, in which thepolymer is formed without the loss of other materials. Polymerization ofthe monomers can be conducted according to any conventional method suchas bulk polymerization and semi-continuous polymerization.

The synthesis is preferably performed by a controlled radicalpolymerization (CRP) technique. Suitable polymerization techniquesinclude ATRP (atom transfer radical polymerization), RAFT (reversibleaddition-fragmentation chain transfer polymerization), MADIX (reversibleaddition-fragmentation chain transfer process, using a transfer activexanthate), catalytic chain transfer (e.g., using cobalt complexes), GTP(group transfer polymerization), or nitroxide (e.g., TEMPO) mediatedpolymerizations.

In a preferred embodiment, the polymeric dispersant used in thepigmented inkjet ink is prepared by a post-polymerization modificationwith a chromophore. The chromophore is covalently linked to the polymerbackbone of the polymeric dispersant. The post-polymerizationmodification can be any suitable reaction, e.g., an esterificationreaction.

An esterification reaction suitable for post-polymerization modificationcan be performed using N,N′-carbonyldiimidazole (CDI). In a first step,the carboxylic moieties of the polymer are activated with CDI to form anintermediate imidazole, which is then esterified with the chromophorehaving a reactive hydroxyl group. The completion of the first step canbe observed when the CO₂ degassing stops. Synthesis scheme with Rrepresenting the chromophore group:

Side products of the reaction can be removed by acidifying the aqueousmedium used for precipitation of the polymer (hydrolysis of remainingactivated esters and protonation of imidazole that remains in water, inthis way achieving the separation from the polymer). If the finalmodified polymer is water-soluble (e.g., modified homopolymer of acrylicacid), a dialysis can be performed to purify the polymer.

In another preferred embodiment, the polymeric dispersant used in thepigmented inkjet ink is prepared by a copolymerization with a monomercontaining a chromophore group. It was observed that the pigment-basedmonomers containing a chromophore group were stable in the presence ofradicals. Classical free radical polymerization (FRP) techniques forpreparing statistical copolymers in a one reactor polymerization andATRP for preparing block copolymers were possible to prepare thepolymeric dispersant used in the present invention.

Monomers with a Chromophore Group

A monomer with a chromophore group for the preparation of the polymericdispersant used in the pigmented inkjet ink according to a preferredembodiment of the present invention can be represented by the generalformula:

A-L-B

whereinA represents a polymerizable functional group, preferably anethylenically unsaturated polymerizable functional group;L represents a divalent linking group; andB represents a chromophore group.

In a preferred embodiment, the ethylenically unsaturated polymerizablegroup is selected from the group consisting of a styrene, an acrylate, amethacrylate, an acrylamide, a methacrylamide, a maleimide, a vinylester, and a vinyl ether.

The monomer with a chromophore group can be represented by Formula(GEN-I):

whereinAR₁, and AR₂ represent a substituted or unsubstituted aromatic group andR represents a substituted or unsubstituted aliphatic group, with theproviso that one of R, AR₁, and AR₂ has a substituent with apolymerizable functional group, preferably an ethylenically unsaturatedpolymerizable functional group.

In a preferred embodiment, AR₂ of Formula (GEN-I) is replaced by analkyl group, preferably methyl or ethyl.

In another preferred embodiment, AR₂ of Formula (GEN-I) is replaced byan aliphatic substituent with a polymerizable functional group,preferably an ethylenically unsaturated polymerizable functional group.Preferably this aliphatic ethylenically unsaturated polymerizablefunctional group is represented by:

Suitable monomers according to Formula (GEN-I) include the monomersdisclosed in Table 1: styrene derivatives, in Table 2: (meth)acrylateand (meth)acrylamide derivatives, and in Table 3: other polymerizablederivatives.

TABLE 1

TABLE 2

TABLE 3

Dispersion Media

The dispersion medium used in the pigmented inkjet ink according to apreferred embodiment of the present invention is preferably a liquid.The dispersion medium may consist of water and/or organic solvent(s).

If the pigmented inkjet ink is a curable pigmented inkjet ink, waterand/or organic solvent(s) are replaced by one or more monomers and/oroligomers to obtain a liquid dispersion medium. Sometimes, it can beadvantageous to add a small amount of an organic solvent to improve thedissolution of the dispersant. The content of organic solvent should belower than 20 wt % based on the total weight of the pigmented inkjetink.

Suitable organic solvents include alcohols, aromatic hydrocarbons,ketones, esters, aliphatic hydrocarbons, higher fatty acids, carbitols,cellosolves, and higher fatty acid esters. Suitable alcohols include,methanol, ethanol, propanol and 1-butanol, 1-pentanol, 2-butanol, andt.-butanol. Suitable aromatic hydrocarbons include toluene and xylene.Suitable ketones include methyl ethyl ketone, methyl isobutyl ketone,2,4-pentanedione, and hexafluoroacetone. Also, glycol, glycolethers,N-methylpyrrolidone, N,N-dimethylacetamid, and N,N-dimethylformamid maybe used.

Suitable monomers and oligomers can be found in Polymer Handbook, Vol.1+2, 4th Edition, edited by J. BRANDRUP et al., Wiley-Interscience,1999.

Suitable examples of monomers for curable pigmented inkjet inks include:acrylic acid, methacrylic acid, maleic acid (or their salts), and maleicanhydride; alkyl(meth)acrylates (linear, branched, and cycloalkyl) suchas methyl(meth)acrylate, n-butyl(meth)acrylate,tert-butyl(meth)acrylate, cyclohexyl(meth)acrylate, and2-ethylhexyl(meth)acrylate; aryl(meth)acrylates such asbenzyl(meth)acrylate and phenyl(meth)acrylate;hydroxyalkyl(meth)acrylates such as hydroxyethyl(meth)acrylate andhydroxypropyl(meth)acrylate; (meth)acrylates with other types offunctionalities (e.g., oxirane, amino, fluoro, polyethylene oxide, andphosphate-substituted) such as glycidyl(meth)acrylate,dimethylaminoethyl(meth)acrylate, trifluoroethyl acrylate,methoxypolyethyleneglycol (meth)acrylate, andtripropyleneglycol(meth)acrylate phosphate; allyl derivatives such asallyl glycidyl ether; styrenics such as styrene, 4-methylstyrene,4-hydroxystyrene, and 4-acetoxystyrene; (meth)acrylonitrile;(meth)acrylamides (including N-mono and N,N-disubstituted) such asN-benzyl(meth)acrylamide; maleimides such as N-phenyl maleimide,N-benzyl maleimide, and N-ethyl maleimide; vinyl derivatives such asvinylcaprolactam, vinylpyrrolidone, vinylimidazole, vinylnaphthalene,and vinyl halides; vinylethers such as vinylmethyl ether; andvinylesters of carboxylic acids such as vinylacetate and vinylbutyrate.

A combination of monomers, oligomers, and/or prepolymers may also beused. The monomers, oligomers, and/or prepolymers may possess differentdegrees of functionality, and a mixture including combinations of mono-,di-, tri- and higher functionality monomers, oligomers, and/orprepolymers may be used.

For oil based inkjet inks, the dispersion medium can be any suitable oilincluding aromatic oils, paraffinic oils, extracted paraffinic oils,naphthenic oils, extracted napthenic oils, hydrotreated light or heavyoils, vegetable oils and derivatives and mixtures thereof. Paraffinicoils can be normal paraffin types (octane and higher alkanes),isoparaffins (isooctane and higher iso-alkanes) and cycloparaffins(cyclooctane and higher cyclo-alkanes) and mixtures of paraffin oils.

Surfactants

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may contain at least one surfactant. The surfactant(s)can be anionic, cationic, non-ionic, or zwitter-ionic and are usuallyadded in a total quantity less than 20 wt % based on the total weight ofthe pigmented inkjet ink and particularly in a total less than 10 wt %based on the total weight of the pigmented inkjet ink.

Suitable surfactants for the pigmented inkjet ink according to apreferred embodiment of the present invention include fatty acid salts,ester salts of a higher alcohol, alkylbenzene sulphonate salts,sulphosuccinate ester salts and phosphate ester salts of a higheralcohol (for example, sodium dodecylbenzenesulphonate and sodiumdioctylsulphosuccinate), ethylene oxide adducts of a higher alcohol,ethylene oxide adducts of an alkylphenol, ethylene oxide adducts of apolyhydric alcohol fatty acid ester, and acetylene glycol and ethyleneoxide adducts thereof (for example, polyoxyethylene nonylphenyl ether,and SURFYNOL™ 104, 104H, 440, 465, and TG available from AIR PRODUCTS &CHEMICALS INC.).

Biocides

Suitable biocides for the pigmented inkjet ink include sodiumdehydroacetate, 2-phenoxyethanol, sodium benzoate, sodiumpyridinethion-1-oxide, ethyl p-hydroxybenzoate, and1,2-benzisothiazolin-3-one and salts thereof.

Preferred biocides are Bronidox™ available from HENKEL and Proxel™ GXLavailable from AVECIA.

A biocide is preferably added in an amount of 0.001 wt % to 3 wt %, morepreferably 0.01 wt % to 1.00 wt %, each based on the total weight of thepigmented inkjet ink.

pH Adjusters

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may contain at least one pH adjuster. Suitable pHadjusters include NaOH, KOH, NEt₃, NH₃, HCl, HNO₃, H₂SO₄, and(poly)alkanolamines such as triethanolamine and2-amino-2-methyl-1-propanol. Preferred pH adjusters are NaOH and H₂SO₄.

Humectants/Penetrants

Suitable humectants include triacetin, N-methyl-2-pyrrolidone, glycerol,urea, thiourea, ethylene urea, alkyl urea, alkyl thiourea, dialkyl ureaand dialkyl thiourea, diols, including ethanediols, propanediols,propanetriols, butanediols, pentanediols, and hexanediols; glycols,including propylene glycol, polypropylene glycol, ethylene glycol,polyethylene glycol, diethylene glycol, tetraethylene glycol, andmixtures and derivatives thereof. Preferred humectants are triethyleneglycol mono butylether, glycerol, and 1,2-hexanediol. The humectant ispreferably added to the inkjet ink formulation in an amount of 0.1 wt %to 40 wt % of the formulation, more preferably 0.1 wt % to 10 wt % ofthe formulation, and most preferably approximately 4.0 wt % to 6.0 wt %.

Preparation of a Pigmented Inkjet Ink

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may be prepared by precipitating or milling thepigment in the dispersion medium in the presence of the polymericdispersant.

Mixing apparatuses may include a pressure kneader, an open kneader, aplanetary mixer, a dissolver, and a Dalton Universal Mixer. Suitablemilling and dispersion apparatuses are a ball mill, a pearl mill, acolloid mill, a high-speed disperser, double rollers, a bead mill, apaint conditioner, and triple rollers. The dispersions may also beprepared using ultrasonic energy.

Many different types of materials may be used as milling media, such asglasses, ceramics, metals, and plastics. In a preferred embodiment, thegrinding media can include particles, preferably substantially sphericalin shape, e.g., beads consisting essentially of a polymeric resin oryttrium stabilized zirconium beads.

In the process of mixing, milling, and dispersion, each process ispreferably performed with cooling to prevent build up of heat.

In the process of mixing, milling, and dispersion, each process isperformed with cooling to prevent build up of heat, and for radiationcurable inkjet inks as much as possible under light conditions in whichactinic radiation has been substantially excluded.

The inkjet ink according to a preferred embodiment of the presentinvention may contain more than one pigment, the inkjet ink may beprepared using separate dispersions for each pigment, or alternativelyseveral pigments may be mixed and co-milled in preparing the dispersion.

The dispersion process can be carried out in a continuous, batch, orsemi-batch mode.

The preferred amounts and ratios of the ingredients of the mill grindwill vary widely depending upon the specific materials and the intendedapplications. The contents of the milling mixture include the mill grindand the milling media. The mill grind includes pigment, polymericdispersant, and a liquid carrier such as water. For inkjet inks, thepigment is usually present in the mill grind at 1 wt % to 50 wt %,excluding the milling media. The weight ratio of pigment over polymericdispersant is preferably 20:1 to 1:2.

The milling time can vary widely and depends upon the pigment,mechanical means and residence conditions selected, the initial anddesired final particle size, etc. In a preferred embodiment of thepresent invention, pigment dispersions with an average particle size ofless than 100 nm may be prepared.

After milling is completed, the milling media is separated from themilled particulate product (in either a dry or liquid dispersion form)using conventional separation techniques, such as by filtration, sievingthrough a mesh screen, and the like. Often the sieve is built into themill, e.g., for a bead mill. The milled pigment concentrate ispreferably separated from the milling media by filtration.

In general, it is desirable to make the inkjet inks in the form of aconcentrated mill grind, which is subsequently diluted to theappropriate concentration for use in the inkjet printing system. Thistechnique permits preparation of a greater quantity of pigmented inkfrom the equipment. By dilution, the inkjet ink is adjusted to thedesired viscosity, surface tension, color, hue, saturation density, andprint area coverage for the particular application.

Spectral Separation Factor

The spectral separation factor SSF was found to be an excellent measureto characterize a pigmented inkjet ink, as it takes into accountproperties related to light-absorption (e.g., wavelength of maximumabsorbance λ_(max), shape of the absorption spectrum andabsorbance-value at λ_(max)) as well as properties related to thedispersion quality and stability.

A measurement of the absorbance at a higher wavelength gives anindication on the shape of the absorption spectrum. The dispersionquality can be evaluated based on the phenomenon of light scatteringinduced by solid particles in solutions. When measured in transmission,light scattering in pigment inks may be detected as an increasedabsorbance at higher wavelengths than the absorbance peak of the actualpigment. The dispersion stability can be evaluated by comparing the SSFbefore and after a heat treatment of, e.g., a week at 80° C.

The spectral separation factor SSF of the ink is calculated by using thedata of the recorded spectrum of an ink solution or a jetted image on asubstrate and comparing the maximum absorbance to the absorbance at ahigher reference wavelength λ_(ref). The spectral separation factor iscalculated as the ratio of the maximum absorbance A_(max) over theabsorbance A_(ref) at a reference wavelength.

${SSF} = \frac{A_{\max}}{A_{ref}}$

The SSF is an excellent tool to design inkjet ink sets with a largecolor gamut. Often, inkjet ink sets are now commercialized, wherein thedifferent inks are not sufficiently matched with each other. Forexample, the combined absorption of all inks does not give a completeabsorption over the whole visible spectrum, e.g., “gaps” exist betweenthe absorption spectra of the colorants. Another problem is that one inkmight be absorbing in the range of another ink. The resulting colorgamut of these inkjet ink sets is low or mediocre.

EXAMPLES Materials

All materials used in the following examples were readily available fromstandard sources such as Aldrich Chemical Co. (Belgium) and Acros(Belgium) unless otherwise specified. The water used was deionizedwater.

SMA 1000P is a styrene maleic anhydride alternating copolymer availablefrom ATOFINA.

VERSICOL E5 was obtained from ALLIED COLLOIDS MANUFACTURING CO LTD as a25% wt solution of pAA in water. This solution was freeze-dried toafford the dry powder of polyacrylic acid that was subsequently used formodification reactions.Raney Nickel™ is a catalysator from DEGUSSA.WAKO V-601 is dimethyl 2,2′-azobisisobutyrate from Wako Pure ChemicalIndustries, Ltd.MSTY or alpha methylstyrene dimer is 2,4-diphenyl-4-methyl-1-pentenefrom Goi Chemical Co., Ltd.AA is acrylic acid from ACROS.MAA is methacrylic acid from ACROS.BA is butyl acrylate from ACROS.EHA is 2-ethyl hexyl acrylate from ACROS.STY is styrene from ACROS.Proxel™ Ultra 5 from AVECIA.Glycerol from ACROS.1,2-propanediol from CALDIC CHEMIE NV.Surfynol™ 104H from AIR PRODUCTS & CHEMICALS INC.PY12 is the abbreviation for C.I. Pigment Yellow 12 for which Permanent™Yellow DHG from CLARIANT was used.PY13 is the abbreviation for C.I. Pigment Yellow 13 for which Irgalite™Yellow BAW from CIBA was usedPY14 is the abbreviation for C.I. Pigment Yellow 14 for which SunbriteYellow 14/274-2168 from SUN CHEMICAL was usedPY17 is the abbreviation for C.I. Pigment Yellow 17 for which Graphtol™Yellow GG from CLARIANT was used

The chemical structure of the color pigments used is listed in Table 4.

TABLE 4 PY12

PY13

PY14

PY17

Measurement Methods 1. Measurement of SSF

The spectral separation factor SSF of the ink was calculated by usingthe data of the recorded spectrum of an ink solution and comparing themaximum absorbance to the absorbance at a reference wavelength. Thechoice of this reference wavelength is dependent on the pigment(s) used:

-   -   if the color ink has a maximum absorbance A_(max) between 400 nm        and 500 nm, then the absorbance A_(ref) must be determined at a        reference wavelength of 600 nm;    -   if the color ink has a maximum absorbance A_(max) between 500 nm        and 600 nm, then the absorbance A_(ref) must be determined at a        reference wavelength of 650 nm;    -   if the color ink has a maximum absorbance A_(max) between 600 nm        and 700 nm, then the absorbance A_(ref) must be determined at a        reference wavelength of 830 nm.

The absorbance was determined in transmission with a Shimadzu UV-2101 PCdouble beam-spectrophotometer. The ink was diluted to have a pigmentconcentration of 0.002%. In the case of a magenta ink, the ink wasdiluted to have a pigment concentration of 0.005%. A spectrophotometricmeasurement of the UV-VIS-NIR absorption spectrum of the diluted ink wasperformed in transmission-mode with a double beam-spectrophotometerusing the settings of Table 5. Quartz cells with a path length of 10 mmwere used and water was chosen as a blank.

TABLE 5 Mode Absorbance Wavelength range 240-900 nm Slit width 2.0 nmScan interval 1.0 nm Scan speed Fast (1165 nm/min) Detectorphoto-multiplier (UV-VIS)

Efficient pigmented inkjet inks exhibiting a narrow absorption spectrumand a high maximum absorbance have a value for SSF of at least 30.

2. Dispersion Stability

The dispersion stability was evaluated by comparing the SSF before andafter a heat treatment of one week at 80° C. Pigmented inkjet inksexhibiting good dispersion stability have a SSF after heat treatmentstill larger than 30.

3. Polymer Analysis

Unless otherwise specified, all polymers have been characterized withgel permeation chromatography (GPC) and nuclear magnetic resonancespectroscopy (NMR) using the following methods. Random or blockcopolymers were analyzed with NMR by dissolving them in a deuteratedsolvent. For ¹H-NMR±20 mg polymer was dissolved in 0.8 mL CDCl₃ orDMSO-d6 or acetonitrile-d3 or D₂O (with or without NaOD addition).Spectra were recorded on a Varian Inova 400 MHz instrument equipped withan ID-probe. For ¹³C-NMR±200 mg polymer was dissolved in 0.8 mL CDCl₃ orDMSO-d6 or acetonitrile-d3 or D₂O (with or without NaOD addition).Spectra were recorded on a Varian Gemini2000 300 MHz equipped with aSW-probe.

Mn, Mw, and polydispersity (PD) values were determined using gelpermeation chromatography. For polymers dissolvable in organic solventsPL-mixed B columns (Polymer Laboratories Ltd) were used with THF+5%acetic acid as mobile phase using polystyrene with known molecularweights as calibration standards. These polymers were dissolved in themobile phase at a concentration of 1 mg/mL. For polymers dissolvable inwater PL Aquagel OH-60, OH-50, OH-40, and/or OH-30 (Polymer LaboratoriesLtd) column combinations were used depending on the molecular weightregion of the polymers under investigation. As mobile phasewater/methanol mixtures adjusted to pH 9.2 with, e.g., disodiumhydrogenphosphate were used with or without the addition of neutral salts, e.g.,sodium nitrate. As calibration standards polyacrylic acids with knownmolecular weights were used. The polymers were dissolved in either wateror water made basic with ammonium hydroxide at a concentration of 1mg/mL. Refractive index detection was used.

An example is now given to illustrate the calculation of the averagecomposition of a random (=statistical) copolymer P(MAA-c-EHA).

The Mn of the copolymer was determined with GPC to be 5000.The molar percentage of each monomer type by NMR was determined to be:45 mole % MAA and 55 mole % EHA.

Calculation:

(0.45×M _(MAA))+(0.55×M _(EHA))=140.09

5000/140.09=total number of monomeric units in average polymer chain=36

Average number of MAA units=0.45×(5000/140.09)=16 units

Average number of EHA units=0.55×(5000/140.09)=20 units

Thus, the average composition is P(MAA₁₆-c-EHA₂₀)

4. Particle Size

The particle size of pigment particles in the pigmented inkjet ink wasdetermined by photon correlation spectroscopy at a wavelength of 633 nmwith a 4 mW HeNe laser on a diluted sample of the pigmented inkjet ink.The particle size analyzer used was a Malvern™ nano-S available fromGoffin-Meyvis.

The sample was prepared by addition of one drop of ink to a cuvettecontaining 1.5 mL water and mixed until a homogenous sample wasobtained. The measured particle size is the average value of 3consecutive measurements consisting of 6 runs of 20 seconds. For goodink jet characteristics (jetting characteristics and print quality) theaverage particle size of the dispersed particles is preferably below 150nm.

5. Calculation of % MW

The % MW is calculated as the ratio of the molecular weight of thepending chromophore group over the molecular weight of the color pigmentmultiplied by 100.

Example 1

This example illustrates that different pigments for inkjet inks can bedispersed using the same polymeric dispersant having one or more pendingchromophore groups similar to the pigments. The polymeric backbone ofthe dispersant is a homopolymer or an alternating polymer, which areknown to have poor dispersing capability. It is also shown that thepolymeric dispersants can be obtained by uncomplicated synthesis

Polymeric Dispersants DISP-1 and DISP-2

VERSICOL E5, a homopolymer of acrylic acid was used as polymericdispersant DISP-1. The polymeric dispersant DISP-2 was prepared bymodifying VERSICOL E5 through esterification with the chromophore MC-2.

Chromophore MC-2

The formation of the chromophore MC-2 was accomplished by diazotation ofcompound MC-1D and subsequent coupling in the compound MC-2B.

Preparation of Compound MC-1C

The vessel used to carry out this reaction was a 3 necked flask equippedwith a stirrer, a cooler, and a dropping-funnel. To a solution of 13.9 g(0.1 mol) 2-nitrophenol (compound MC-1A) in 100 mL dimethylformamide wasadded 31.8 g (0.3 mol) of sodiumcarbonate. The mixture was heated to atemperature of about 150-160° C. and 16.1 g (0.2 mol) of 2-chloroethanol(compound MC-1B) was added drop-wise. After addition of the2-chloroethanol, the temperature was maintained at a temperature between150° C. and 160° C. for about 7 hours. The charge was cooled whilestirring and the formed inorganic salts were filtered off. The filtratewas concentrated by evaporation at a temperature of 40° C. until a redcolored mixture of oil and solid was formed. Then the oil was dissolvedin methylenechloride and the inorganic salts were filtered off. Thefiltrate was evaporated for a second time and the formed yellow oil waspurified by preparative column chromatography. The yield of compoundMC-1C was 79%.

Synthesis Scheme of Compound MC-1C:

Preparation of Compound MC-1D

Compound MC-1D was made by catalytic reduction of compound MC-1C withhydrogen. A reactor was filled with 18.3 g (0.1 mol) of compound MC-1Cin 100 mL ethanol and 1 mL of Raney Nickel™ slurry was added. The volumeof the mixture was set to 150 mL with ethanol and the reduction wascarried out at a starting temperature of 35° C. under an initialH₂-pressure of 60 bar. By shaking the reactor, the exothermic reactionstarted and the temperature increased to about 60° C. After reduction,the charge was mixed during 1 hour and the Raney Nickel™ was filteredoff. The filtrate was evaporated at a temperature of 50° C. until thedesired white crystalline product MC-1D appeared. The yield of compoundMC-1D was 95%.

Synthesis Scheme of Compound MC-1D:

Preparation of Chromophore MC-2

29.98 mL (0.36 mol) of concentrated hydrochloric acid was added to asuspension of 15.3 g (0.1 mol) of compound MC-1D in 300 mL water. Thismixture was cooled to a temperature of about 0° C. to 5° C. and 8.97 g(0.13 mol) of sodiumnitrite was added. The diazonium-salt was kept at atemperature between 0° C. and 5° C. After 15 minutes, the excess ofnitrite was neutralized by adding 3.0 g (0.03 mol) of sulfamic acid anda pH of 7 was obtained by adding 25.2 g (0.3 mol) of sodiumcarbonate.While the diazionium-salt was made, 20.7 g (0.1 mol) of MC-2B from ACROSwas dissolved in a mixture of 500 mL methanol and 10.0 mL (0.1 mol) 29%sodiumhydroxide-solution. This solution was added drop-wise into thediazonium-salt solution and a yellow suspension was immediately formed.The temperature was maintained between 0 and 5° C. for about 3 hours andthe yellow product MC-2 was filtered and washed with methanol. The yieldof the chromophore MC-2 was 92%.

Synthesis Scheme of the Chromophore MC-2:

Synthesis of Polymeric Dispersant DISP-2

The polymeric dispersant DISP-2 was prepared by modifying DISP-1(VERSICOL E5) with the chromophore MC-2. The resulting pendingchromophore group PC-2 was linked by C* to the polymeric backbonethrough a linking group L containing an ester bond.

The reaction was performed in a three-necked round-bottomed flask thatwas equipped with a stirrer, a cooler, and a bubble-counter on top. 4 gof the polyacrylic acid homopolymer (VERSICOL E5 as powder form) wasintroduced in the flask and dissolved in 40 mL of anhydrousdimethylacetamide (DMA). A slight flux of nitrogen was circulatedthrough the flask. After (VERSICOL E5) was dissolved, 4.48 g of1,1′-carbonyldiimidazole (CDI) was added and CO₂ evolution was observed.The reaction was further stirred at room temperature for 1 hour afterwhich 4.10 g of the chromophore MC-2 in combination with 169 mg of thecatalyst dimethylaminopyridine (DMAP) were added. The heterogeneousmixture was stirred and heated to 80° C. for 20 hours. The reactionmixture was cooled to room temperature and was treated by slowly adding10 mL of a 2% v/v of acetic acid/water solution.

The heterogeneous mixture obtained was basified to pH 10 with NaOH andfiltrated to remove remaining precipitates. The solution was dialyzed inwater (Regenerated Cellulose Dialysis Membrane of MWCO of 1,000 Dalton—SPECTRA/POR™ 6) for two days and precipitates that appeared werefiltered off again. The obtained solution was freeze-dried to give afluffy yellow powder. Yield of DISP-2 was 4.8 g.

Analytical results of DISP-2: GPC: Mn=1208; Mw=4960; PD=4.11

(aqueous GPC; calibrated vs. PAA-standards)The degree of substitution was determined by ¹H-NMR-spectroscopy andexpressed as the percentage of AA monomeric units. The degree ofsubstitution of DISP-2 with the chromophore MC-2 was 13 molar %.

Polymeric Dispersants DISP-3 to DISP-6

The alternating copolymer SMA 1000P was used as polymeric dispersantDISP-3 to prepare comparative inkjet inks. DISP-3 was then used forpreparing styrene maleic acid copolymers modified by chromophores MC-7and MC-12.

Chromophore MC-7

The synthesis of the chromophore MC-7 will now be described.

Preparation of Chromophore MC-7

6.9 g (45.2 mmol) of compound MC-1D was mixed with 40 mL H₂O and 10 mLmethanol. Then, 4.5 g (54 mmol) of compound MC-1F was added and themixture was stirred for 30 minutes. This is mixture A-MC-7. 5.8 g (45.2mmol) of compound MC-7A was mixed in 150 mL H₂0. 16.2 g (162 mmol) ofconcentrated HCl was added and the mixture was then cooled to atemperature of about 0° C. to 5° C. 4.05 g (58.8 mmol) of sodium nitritewas added and the mixture was kept at a temperature between 0° C. and 5°C. After 15 minutes, the excess of nitrite was neutralized by adding1.36 g (13.6 mmol) of sulfamic acid and a pH of 7 was obtained by adding11.4 g (136 mmol) of sodium carbonate. The mixture A-MC-7 was added andthe mixture was stirred for 1 hour at a temperature between 0° C. and 5°C. The yellow product was filtered and washed with methanol. The yieldof the chromophore MC-7 was 80%.

Synthesis Scheme of Chromophore MC-7:

Chromophore MC-12

The synthesis of the chromophore MC-12 will now be described.

Preparation of Compound MC-8B

The vessel used to carry out this reaction was a 3 necked flask equippedwith a stirrer, a cooler and a dropping-funnel. To a solution of 140 g(1 mol) 3-nitrophenol (compound MC-8A) and 1.4 L N-methylpyrolidone wasadded 190 mL sodium methylate 30% (1.025 mol). The mixture wasdestillated at a temperature of 100° C. and 80 mbar pressure. After thedestination 87 mL (1.3 mol) of 2-chloroethanol (compound MC-1B) wasadded dropwise. After addition of the 2-chloroethanol, the mixture washeated to a temperature of about 120° C. for 3 hours. The reactionmixture was poured into 6 L of water with 85 mL HCl conc. The productwas filtrated. The yield of compound MC-8B was 27%.

Synthesis Scheme of Compound MC-8B:

Preparation of Compound MC-8C

Compound MC-8C was made by catalytic reduction of compound MC-8B withhydrogen. A reactor was filled with 101 g (0.55 mol) of compound MC-8Bin 700 mL ethanol and 11 mL of Raney Nickel™ slurry was added. Thereduction was carried out at a starting temperature of 75° C. under aninitial H₂-pressure of 46 bar. After reduction, the charge was mixedduring 1 hour and the Raney Nickel™ was filtered off. The filtrate wasevaporated at a temperature of 50° C. until the desired whitecrystalline product MC-8C appeared. The yield of compound MC-8C was 95%.

Synthesis Scheme of Compound MC-8C:

Preparation of Chromophore MC-12

6.9 g (45.2 mmol) of compound MC-8C was mixed with 40 mL H₂O and 10 mLmethanol. Then, 4.5 g (54 mmol) of compound MC-1F was added and themixture was stirred for 30 minutes. This is mixture A-MC-12. 5.8 g (45.2mmol) of compound MC-7A was added to a mixture of 15 mL H₂0 and 50 mLmethanol. 16.2 g (162 mmol) of concentrated HCl was added. The solutionwas cooled to a temperature of about 0° C. to 5° C. 4.05 g (58.8 mmol)of sodium nitrite was added and the mixture was kept at a temperaturebetween 0° C. and 5° C. After 15 minutes, the excess of nitrite wasneutralized by adding 1.36 g (13.6 mmol) of sulfamic acid and a pH of 7was obtained by adding 11.4 g (136 mmol) of sodium carbonate. Themixture A-MC-12 was added and the mixture was stirred for 1 hour at atemperature between 0° C. and 5° C. The stirring was continued for 1hour at a temperature of 20° C. The yellow product was filtered andwashed with methanol. The yield of the chromophore MC-12 was 91%.

Synthesis Scheme of the Chromophore MC-12:

Synthesis of Polymeric Dispersants DISP-3 and DISP-4

The alternating copolymer SMA 1000P (also used below as DISP-3 toprepare comparative inkjet inks) was used for preparing styrene maleicacid copolymers modified by the chromophore MC-7. The resulting pendingchromophore group PC-7, consisting of 35 atoms, was linked by C* to thepolymeric backbone through a linking group L containing an ester bond;

The synthesis is based on a kinetic study on the mono-esterification ofstyrene maleic anhydride copolymers disclosed by HU, et al.Monoesterification of Styrene-Maleic Anhydride Copolymers with Alcoholsin Ethylbenzene: Catalysis and Kinetics, Journal of Polymer Science,Part A, Polymer Chemistry, 1993, Vol. 31, pp. 691-670.

The styrene maleic anhydride copolymer SMA 1000P was dissolved in a 1/1solvent mixture of DMA (dimethylacetamide) and MEK. Then 5 mole % ofMC-7, based on the anhydride units in the polymer, was added and thereaction was allowed to continue for 24 hours at room temperature. After24 hours, the polymer was precipitated with methyl t-butyl ether,isolated by filtration and washed several times with methyl t-butylether. The isolated polymer was suspended in water and the pH wasadjusted to 10, using 5 N NaOH. The mixture was stirred until thepolymer dissolved. Upon complete dissolution, the pH was adjusted to 2,using 6N HCl. The polymeric dispersant was precipitated from the medium,isolated by filtration and dried. The degree of substitution wasdetermined by ¹H-NMR-spectroscopy and expressed as the percentage ofesterified maleic acid units. The degree of substitution of DISP-4 withthe chromophore MC-7 was 2%.

Synthesis Scheme:

Synthesis of Polymeric Dispersant DISP-5

The polymeric dispersant DISP-5 was prepared in the same manner asDISP-4 except that 20 mole % was used instead of 5 mole % of MC-7, basedon the anhydride units in the polymer. The degree of substitution ofDISP-5 with the chromophore MC-7 was found to be 7%.

Synthesis of Polymeric Dispersant DISP-6

The polymeric dispersant DISP-6 was prepared by modifying thealternating copolymer SMA 1000P (DISP-3) through esterification with thechromophore MC-12.

The pending chromophore group PC-12 of DISP-6 is structurally the sameas the pending chromophore group PC-7 of the polymeric dispersantDISP-4, except that it is connected through the same linking group L bya different C* to the polymeric backbone of DISP-6, i.e. linked in parainstead of ortho position.

The polymeric dispersant DISP-6 was prepared in the same manner asDISP-4 except that the chromophore MC-7 was replaced by MC-12. Thedegree of substitution of DISP-6 with the chromophore MC-12 was found tobe 2%.

Polymeric Dispersants DISP-7 and DISP-8

The polymeric dispersants DISP-7 and DISP-8 were prepared bycopolymerizing a monomer MONC already containing the chromophore groupPC-2.

Synthesis of the Monomer MONC

Ethylacetate (480 ml) was cooled to 0° C. Acrylic acid (19.0 g, 0.264mol) and 2,6-di-tert-butyl-4-methylphenol (0.2 g, 0.00088 mol) wereadded. Triethylamine (26.7 g, 0.264 mol) was added drop-wise while thetemperature was maintained between −5° C. and 0° C. Finally benzenesulfonyl chloride (22.3 g, 0.126 mol) was added drop-wise. Triethylaminehydrochloride precipitated. The reaction mixture was allowed to stir for1 hour at 0° C. resulting in the formation of the symmetric anhydride.To this mixture N-hydroxysuccinimide (0.7 g, 0.006 mol) and MC-2 (22.3g, 0.06 mol) were added at 5° C. The reaction mixture was refluxed (78°C.) for about 17 hours. The reaction mixture was diluted with EtOAc (100ml) and extracted with distilled water (400 ml). The organic layer wasseparated and again extracted with a mixture of an aqueous solution ofhydrochloric acid and distilled water (⅕). Finally the organic layer waswashed with water and dried over MgSO₄. After evaporation of thesolvent, the residue was suspended into distilled water and stirred for45 minutes. Filtration provided a yellow solid.

Synthesis Scheme of MONC:

Synthesis of Polymeric Dispersant DISP-7

The polymeric dispersant DISP-7 was prepared by copolymerizing themonomer MONC with AA monomers in 90/10 molar ratio of AA/MONC.

The synthesis was performed in a 250 mL three-necked round-bottomedflask which was equipped with a cooling unit, a bubble counter on topand a stirring bar. 6.04 g of the monomer AA, 3.96 g of the monomerMONC, 0.43 g of the initiator WAKO™ V601, 0.44 g of the transfer agentMSTY were introduced in 89.13 g of dioxane. The total weight %concentration of the monomers was 10. The reaction mixture was degassedby bubbling nitrogen in the solution for approximately 30 min. The flaskwas immersed into an oil bath and heated to 80° C. and the mixture wasfurther reacted for 20 hours. After polymerization, the reaction mixturewas cooled down to room temperature. The polymer was precipitated in1000 mL of water (acidified with HCl) and filtered off, followed bydrying under vacuum at 40° C. for 24 hours to afford 8.4 g of yellowpowder of DISP-7. (Yield=77.2%)

Analytical results of DISP-7: GPC: Mn=2745; Mw=5478; PD=2.00

(GPC in THF+5% acetic acid; calibrated vs. PS-standards)NMR: AA/MONC molar ratio was 90/10. On average DISP-7 contained 23.01 AAmonomeric units and 2.55 MONC monomeric units.

Synthesis of Polymeric Dispersant DISP-8

The polymeric dispersant DISP-8 was prepared by copolymerizing themonomer MONC with AA monomers in 71/29 molar ratio of AA/MONC.

The synthesis was performed in a 100 mL three-necked round-bottomedflask which was equipped with a cooling unit, a bubble counter on topand a stirring bar. 1.42 g of the monomer AA, 3.58 g of the monomerMONC, 0.21 g of the initiator WAKO™ V601, 0.22 g of the transfer agentMSTY were introduced in 44.57 g of dioxane. The total weight %concentration of the monomers was 10. The reaction mixture was degassedby bubbling nitrogen in the solution for approximately 30 min. The flaskwas immersed into an oil bath and heated to 80° C. and the mixture wasfurther reacted for 20 hours. After polymerization, the reaction mixturewas cooled down to room temperature. The polymer was precipitated in 500mL of water (acidified with HCl) and filtered off, followed by dryingunder vacuum at 40° C. for 24 hours to afford 4.6 g of yellow powder ofDISP-8. (Yield=84.7%)

Analytical results of DISP-8: GPC: Mn=2633; Mw=3982; PD=1.51

(GPC in THF+5% acetic acid; calibrated vs. PS-standards)

NMR: AA/MONC molar ratio was 71/29. On average DISP-8 contained 10.71 AAmonomeric units and 4.37 MONC monomeric units.

Preparation of Inkjet Ink

All inkjet inks were prepared in the same manner to obtain a compositionas described in Table 6, except that different pigments and dispersantswere used.

TABLE 6 Component wt % Pigment 4.00 Dispersant 2.40 1,2-propanediol21.00 Glycerol 7.00 Proxel ™ Ultra 5 0.80 Surfynol ™ 104H 0.09 Water64.71

An ink composition was made by mixing the pigment, the dispersant, andabout half of the water with a dissolver and subsequently treating thismixture with a roller mill procedure using yttrium-stabilised zirconiumoxide-beads of 0.4 mm diameter (“high wear resistant zirconia grindingmedia” from TOSOH Co.). A polyethylene flask of 60 mL was filled to halfits volume with grinding beads and 20 g of the mixture. The flask wasclosed with a lid and put on the roller mill for three days. The speedwas set at 150 rpm. After milling the dispersion was separated from thebeads using a filter cloth. During stirring, the surfactant Surfynol™104H and the biocide Proxel™ Ultra 5, glycerol, 1,2-propanediol and theremaining water were added. This mixture was stirred for 10 minutes andfiltered. The filtration was performed in two steps. First, the inkmixture is filtered using a (plastipak) syringe with a microfiberdisposable filter capsule with 1 μm pore diameter (GF/B microfiber fromWhatman Inc.) Then the same procedure is repeated on the filtrate. Afterthe second filtration the ink is ready for evaluation.

Using the above method, the comparative inkjet inks COMP-1 to COMP-2 andthe inventive inkjet inks INV-1 to INV-7 were prepared according toTable 7.

TABLE 7 Polymeric Polymeric Chromophore Color Inkjet Ink Dispersantbackbone Group Pigment COMP-1 DISP-1 Homopolymer None PY17 INV-1 DISP-2Homopolymer PC-2 PY17 COMP-2 DISP-3 Alternating None PY17 copolymerINV-2 DISP-4 Alternating PC-7 PY17 copolymer INV-3 DISP-5 AlternatingPC-7 PY17 copolymer INV-4 DISP-6 Alternating PC-12 PY17 copolymer INV-5DISP-6 Alternating PC-12 PY12 copolymer INV-6 DISP-6 Alternating PC-12PY13 copolymer INV-7 DISP-6 Alternating PC-12 PY14 copolymer INV-8DISP-7 Homopolymer PC-2 PY17 INV-9 DISP-8 Homopolymer PC-2 PY17

Results and Evaluation

The spectral separation factor (SSF) was determined for each sampledirectly after preparation. The results are listed in Table 8 togetherwith the % MW.

TABLE 8 Inkjet Chromophore Color Ink Group Pigment % MW SSF COMP-1 NonePY17 0 0 INV-1 PC-2 PY17 45 79 COMP-2 None PY17 0 10 INV-2 PC-7 PY17 46107 INV-3 PC-7 PY17 46 65 INV-4 PC-12 PY17 46 38 INV-5 PC-12 PY12 50 48INV-6 PC-12 PY13 46 49 INV-7 PC-12 PY14 48 49 INV-8 PC-2 PY17 45 234INV-9 PC-2 PY17 45 254

From Table 8, it is clear that, although a homopolymer or an alternatingpolymer was used as the polymeric backbone for the dispersant, in theinventive pigmented inkjet inks INV-1 to INV-7 the pigments weredispersed exhibiting a high dispersion quality. The SSF of the inventiveyellow inkjet inks INV-1 to INV-7 all were larger than 30 and thereforecan be used to compose inkjet ink sets with high color gamut.Furthermore, one can see that the same polymeric dispersant with apending chromofore group can be used for different pigments. Theinventive pigmented inkjet inks INV-8 and INV-9 illustrate that thesimple method of copolymerizing a monomer already containing a smallchromophore group offers the advantage of well controlled design andsynthesis of polymeric dispersants for one or more pigments for aspecific dispersion medium.

Example 2

This example illustrates the improved thermal stability of inkjet inksusing a polymeric dispersant in accordance with preferred embodiments ofthe present invention for homopolymer and a statistical copolymer.

Polymeric Dispersants DISP-9 and DISP-10

A statistical copolymer of AA and STY was prepared as polymericdispersant DISP-9. The polymeric dispersant DISP-10 was prepared bymodifying the polymeric dispersant DISP-9 with the chromophore MC-2.

Synthesis of Polymeric Dispersant DISP-9

The synthesis was performed in a 250 ml three-necked round bottomedflask which was equipped with a cooling unit, a bubble counter on topand a stirring bar. 18.40 g of the monomer AA, 26.60 g of the monomerSTY, 2.65 g of the initiator WAKO™ V601, 2.72 g of the transfer agentalpha-methylstyrene dimer were introduced in 99.64 g of isopropanol. Thetotal weight % concentration of the monomers was 30. The reactionmixture was degassed by bubbling nitrogen in the solution forapproximately 30 min. The flask was immersed into an oil bath and thenheated to 80° C. and the mixture was further reacted for 20 hours. Afterpolymerization, the reaction mixture was cooled down to roomtemperature. The polymer was precipitated in 1.5 L of water followed bydrying under vacuum at 30° C. for 24 hours to afford 39.89 g of whitepowder of DISP-9. (Yield=79.20%)

Analytical results of DISP-11: GPC: Mn=4425; Mw=8248; PD=1.86 (GPC inTHF+5% acetic acid; calibrated vs. PS-standards) NMR: AA/STY molar ratiowas 57/43. On average DISP-9 contains 29 AA monomeric units and 22 STYmonomeric units.

Synthesis of Polymeric Dispersant DISP-10

The polymeric dispersant DISP-10 was prepared by modifying the polymericdispersant DISP-9 with the chromophore MC-2.

The reaction was performed in a three-necked round-bottomed flask thatwas equipped with a stirrer, a cooler, and a bubble-counter on top. 4.25g of the p(AA-co-St) copolymer (DISP-9) was introduced in the flask anddissolved in 20 ml of anhydrous dimethylacetamide (DMA). A slight fluxof nitrogen was circulated through the flask. After DISP-9 wasdissolved, 2.18 g of 1,1′-carbonyldiimidazole (CDI) was added and CO₂evolution was observed. The reaction was further stirred at roomtemperature for 1 hour after which 1 g of the chromophore MC-2 incombination with 82.2 mg of the catalyst dimethylaminopyridine (DMAP)were added. The heterogeneous mixture was stirred until a clear solutionwas obtained. The solution was heated to 80° C. for 20 hours. Thereaction mixture was cooled to room temperature and was treated byslowly adding 10 ml of a 2% v/v of acetic acid/water solution. Thepolymer was then precipitated in 300 ml of water, filtered off and driedunder vacuum to give a yellow powder. Yield of DISP-10 was 4.66 g.

Analytical results of DISP-10: GPC: Mn=4128; Mw=7027; PD=1.70 (GPC inTHF+5% acetic acid; calibrated vs. PS-standards)

The degree of substitution was determined by ¹H-NMR-spectroscopy andexpressed as the percentage of AA monomeric units. The degree ofsubstitution of DISP-10 with the chromophore MC-2 was 11 molar %.

Preparation of Inkjet Inks

The comparative inkjet inks COMP-3 and COMP-4 and the inventive inkjetinks INV-10 and INV-11 were prepared in the same manner as in EXAMPLE 1using the pigments and polymeric dispersants according to Table 9.

TABLE 9 Inkjet Polymeric Chromophore Color Ink Dispersant Group Pigment% MW COMP-3 DISP-1 None PY17 0 INV-10 DISP-2 PC-2 PY17 81 COMP-5 DISP-9None PY17 0 INV-11 DISP-10 PC-2 PY17 81

Results and Evaluation

The spectral separation factor (SSF) was determined for each sampledirectly after preparation and was determined again after a severe heattreatment of 1 week at 80° C. The results are listed in Table 10together with the % MW.

TABLE 10 SSF Inkjet Chromophore Color 1 week Ink Group Pigment % MW SSF80° C. COMP-3 None PY17 0 0 0 INV-10 PC-2 PY17 81 79 53 COMP-4 None PY170 37 42 INV-11 PC-2 PY17 81 55 58

From Table 10, it is clear that the inventive pigmented inkjet inksINV-10 and INV-11 were capable of dispersing the C.I. Pigment Yellow 17with high dispersion quality and at least enhanced dispersion stability.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1-12. (canceled)
 13. A pigment dispersion comprising a color pigmentrepresented by formula (I):

a polymeric dispersant having, via a linking group covalently linked toits polymeric backbone, at least one pending chromophore group which hasa molecular weight smaller than 90% of the molecular weight of the colorpigment; wherein X1 to X4 are independently selected from the groupconsisting of hydrogen and a halogen atom; R1 to R10 are independentlyselected from the group consisting of hydrogen, a halogen atom, a methylgroup, an ethyl group, a methoxy group, and an ethoxy group; the atleast one pending chromophore group is a chromophore group occurring asa side group on the polymeric backbone and not a group in the polymericbackbone itself or occurring solely as an end group of the polymericbackbone; the at least one pending chromophore group is represented byformula (II):

wherein one of L1, L2, or L3 is the linking group and is selected fromthe group consisting of an aliphatic group, a substituted aliphaticgroup, an unsaturated aliphatic group, and a substituted unsaturatedaliphatic group; L1, L2, and/or L3, if not representing the linkinggroup, are independently selected from the group consisting of hydrogen,an alkyl group, an alkenyl group, an alkoxy group, a carboxylic acidgroup, an ester group, an acyl group, a nitro group, and a halogen; AR1and AR2 represent an aromatic group; and n represents the integer 0or
 1. 14. The pigment dispersion according to claim 13, wherein thepending chromophore group is represented by Formula (III):

wherein one of R1 to R11 is the linking group forming a covalent bondwith the polymeric backbone; R1 to R11, if not representing the linkinggroup, are independently selected from the group consisting of hydrogen,an alkyl group, an alkenyl group, an alkoxy group, an alcohol group, acarboxylic acid group, an ester group, an acyl group, a nitro group, anda halogen; or R7 and R8 may together form a heterocyclic ring.
 15. Thepigment dispersion according to claim 14, wherein the heterocyclic ringformed by R7 and R8 is imidazolone or 2,3-dihydroxypyrazine.
 16. Thepigment dispersion according to claim 13, wherein the linking groupcontains at least one atom selected from the group consisting of anoxygen atom, a nitrogen atom, and a sulfur atom.
 17. The pigmentdispersion according to claim 16, wherein the linking group in unreactedform is represented by —O—CH₂—CH₂—OH.
 18. The pigment dispersionaccording to claim 13, wherein the pending chromophore group representedby formula (III) having an unreacted linking group is selected from thegroup consisting of:


19. The pigment dispersion according to claim 13, wherein the colorpigment is selected from the group consisting of C. I. Pigment Yellow12, 13, 14, 17, 55, 63, 81, 83, 87, 113, 121, 124, 152, 170, 171, 172,174, and
 188. 20. The pigment dispersion according to claim 13, whereinthe polymeric backbone of the polymeric dispersant is a homopolymer. 21.The pigment dispersion according to claim 13, wherein the pendingchromophore group is present in the range 1 to 30 percent based on thenumber of monomeric units of the polymeric dispersant.
 22. The pigmentdispersion according to claim 13, wherein the polymeric dispersant has anumber average molecular weight Mn between 500 and 30,000 and apolymeric dispersity PD smaller than
 2. 23. The pigment dispersionaccording to claim 13, wherein the pigment dispersion is a curableinkjet ink.
 24. A method for preparing the inkjet ink according to claim13, comprising the step of preparing the polymeric dispersant bycopolymerizing a monomer already containing the pending chromophoregroup.